The present invention generally relates to a method for making an initially electrically nonconductive surface electrically conductive by cleaning and conditioning the surface and then applying an electrically conductive coating. The present invention relates more particularly to such a method for making the initially-nonconductive through hole and via walls of printed wiring boards electrically conductive, so they can be electroplated. ("Through holes" as used herein refers both to through holes and to vias.)
Conductive graphite and carbon black dispersions are used to provide a superior conductive coating on through hole walls and other nonconductive surfaces. Such dispersions, methods for using such dispersions to coat through holes, and improved printed wiring boards fabricated by using such dispersions are defined in U.S. Pat. Nos. 5,476,580 and 5,389,270, respectively issued to Thorn et al. on Feb. 14, 1995, and Dec. 19, 1995. Both patents referred to in the preceding sentence are incorporated herein by reference in their entireties. A graphite composition, cleaners, conditioners, and other materials and directions needed to practice these patents are available under the trademark SHADOW.RTM. from Electrochemicals Inc., Maple Plain, Minn. Other carbon dispersions containing carbon black or graphite are described, for example, in U.S. Pat. No. 5,139,642.
A problem with "blowholes" occasionally develops after the through hole wall has received a conductive coating, has been electroplated, and is then suddenly heated, as by contacting it with molten solder. (Soldering is carried out by coating the through hole walls and other conductive surfaces of a printed wiring board with hot, molten metal to make electrical connections by wetting and filling the spaces between the conductive through hole surfaces and the leads of electrical components which have been inserted through the through holes. A properly soldered through hole is filled with solder.)
Soldering heats the plated through hole walls very quickly. If there are any gaps or voids in the plated copper, moisture in the substrate is vaporized by the hot solder, which can blow some or all of the solder out of the hole and breach the copper layer. The result is a blowhole or a partially-filled or empty hole, any of which is counted as a soldering defect.
The problem of blowholes in through holes made electrically conductive by electroless plating, and the solution to blowholes when that technology is used, are described in a series of articles published in CIRCUIT WORLD, Vol. 12 No. 4 (1986), Vol. 13 No. 1 (1986), and Vol. 13 Nos. 2-3 (1987), under the common title, Blowholing in PTH Solder Fillets. A related article is C.Lea, The Harmfulness of Blowholes in PTH Soldered Assemblies, CIRCUIT WORLD, Vol.16, No.4, (1990). All the articles in this paragraph are incorporated herein by reference in their entirety for their discussion of blowholes in electroless copper technology.
Recently, the present inventors have discovered that blowholes can occasionally be a problem for electroplated through holes which have been made conductive by applying certain aqueous conductive-carbon-based conductive compositions. Thus, a need has arisen to solve the problem of blowholes when a carbon-based conductive coating is used to make through hole walls electrically conductive to facilitate electroplating.
Separately, printed wiring boards have been exposed to ultrasonic energy to facilitate cleaning through holes. See New Process Forces a Solution, CIRCUITS MANUFACTURING, June 1987, p. 18; F. John Fuchs, Ultrasonic Cleaning, p.145. These articles do not discuss blowholes.